George S Mickelson Trail along Rapid Creek Trail is a 19.4 mile moderately trafficked out and back trail located near Rapid City, SD that features a river and is good for all skill levels. The trail offers a number of activity options and is accessible from April until October.
I did 26 miles on the trail by bike from Hill City--13 one way and 13 back. 55 minutes of a slight incline at moderate difficulty, followed by half hour of flying down the other side. (Turned back.) Then 50 minutes back up at a greater incline (really hard for me at this point), followed by coasting for another hour. It was a peaceful, gorgeous ride going north out of Hill City. Just know what you are getting into. I just hope I can still walk tomorrow.
The entire trail is 108.8 miles along the old railway tracks. Gorgeous, fun and not too difficult... but don't be fooled, the inclines are long and steady and will give you a great workout!